1. Field of the Invention
The present invention relates to the supply of fluid under pressure to the lift cylinder of a dump truck for raising the bed of the truck from its chassis, and to the lowering of the bed at a controlled rate by limiting the discharge of the fluid from the lift cylinder.
2. Description of the Related Art
Hydraulic lift mechanisms for vehicles such as dump trucks are well known. A piston and cylinder are mounted between the chassis and the bed of the truck. A hydraulic pump is connected to a transmission power take-off. The pump receives fluid from a reservoir and supplies the fluid under pressure to the lift cylinder. A slide valve is used to shut off the supply of fluid to the cylinder and bypass the pump after the bed has been raised. To lower the bed, the slide valve is operated to discharge fluid from the cylinder back to the reservoir.
Additional controls have been added to the hydraulic lift mechanism of a dump truck to provide ease of operation and improved safety. Gregory U.S. Pat. No. 4,236,757, for example, discloses the use of a flexible cable and control knob for remotely actuating the slide valve in the hydraulic lift. The control knob is fitted with a safety latch for precluding inadvertent raising or lowering of the truck bed.
Cresci et al. U.S. Pat. No. 3,695,710 discloses a control, in which the fluid under pressure is supplied to the hydraulic cylinder only when the gear shift lever of the transmission is in neutral and the brakes of the vehicle have been applied. When it is desired to lower the truck bed, a solenoid valve is energized to release fluid from the cylinder and return it to a reservoir through a pipe. It is said that a restrictor in the return line reduces the fluid flow so that the piston gradually returns to its lowest position.
Japan 54-120123 discloses a control for the hydraulic lift of a dump truck in which the bed is lowered by draining fluid through a control valve that is throttled in accordance with the size of the dump angle of the bed to relax the impact between the bed and the truck body. The throttle valve is constructed in a housing including a check valve connected in parallel with the throttle valve.
Japan 54-126325 discloses a pneumatic system for indirect operation of the slide valve controlling the hydraulic lift of a dump truck. To lower the hydraulic lift, high pressure air is fed to an air cylinder to push a valve rod into the slide valve. To decrease the speed at which the hydraulic lift is lowered, the rate at which the valve rod is pushed into the slide valve is limited by limiting the flow of exhaust air from the air cylinder. A check valve forces the exhaust air to flow through a throttle valve. Therefore, the throttle valve gradually exhausts the air from the air cylinder so as to elongate the stroke operation time of the slide valve.
Despite the widespread use of hydraulic lift controls, a substantial number of people continue to be killed by the accidental lowering of dump truck beds. This is probably due in part to the use of inexpensive flexible cables for remotely operating the slide valve from the cab of the truck. The flexible cables are subject to wear, corrosion, and overload, and tend to jam or lock. In some of these cases, the operators reach or stand under the raised dump truck body to operate the slide valve with their arms, and are unaware that the truck bed can drop almost instantly. Although this sad situation could be corrected to some degree by the use of better hydraulic lift controls, such controls are more expensive and may introduce additional long term reliability problems. Electrical and pneumatic controls, for example, are subject to potential loss of electrical or pneumatic power. Hydraulic controls such as throttle valves are subject to wear and clogging by particulate contamination.